Western Blot Test

Screening tests (such as ELISA) operate with very high sensitivity to avoid missing any patient; this sometimes causes them to mistake simple flu or pregnancy antibodies for HIV, resulting in a "False Positive." Therefore, 100% confirmation is achieved with the Western Blot by searching for all protein building blocks (bands) of the microbe.

While ELISA tests generally yield results within hours or the same day, the Western Blot requires a much more complex separation and incubation process, so it can take 1 to 2 weeks for the results to be available.

Generally, laboratories store the same blood sample (serum) they used for the screening test (ELISA) and perform the Western Blot on it. Sometimes, to be absolutely sure, the patient may be asked to provide fresh blood.